Radar sensors are often used in driver assistance systems for vehicles, for example in systems for automatic spacing control or in collision warning systems, to detect the traffic environment. In addition to the distance and relative speed, the azimuth angle of localized objects is generally also important since, for example when localizing preceding vehicles, the azimuth angle makes possible a lane allocation. The elevation angle of the localized objects can also be important, since it allows a conclusion as to the relevance of the target, for example whether the target can be driven over or under or is an obstacle representing a potential collision hazard.
The azimuth angles and elevation angles of the targets can be ascertained from amplitudes and/or phase differences of transmitting and/or receiving antennas of an antenna array. in order to improve the accuracy and separation capability of the angle estimate, it is possible to use radar sensors that operate on the multiple input-multiple output (MIMO) principle. Unlike with conventional single input-multiple output (SIMO) radar sensors—which work with one transmitting antenna and multiple receiving antennas—multiple transmitting antenna elements and multiple receiving antenna elements are used. In order to allow the signals of the transmitting antenna elements to be separated at the receiving antenna elements, the transmitted signals must be uncorrelated (orthogonal). This can be achieved by time multiplexing, frequency multiplexing, or code multiplexing.
In the context of angle estimation, the received signals are compared with a previously established angle-dependent antenna diagram. For the case in which only a single target is being localized (or multiple targets that can nevertheless be clearly distinguished from one another based on spacing and relative speed), the estimated angle is obtained as the position of the best agreement between the received signal and the antenna diagram. For the general case of multiple-target estimation, special estimating algorithms are known which supply estimated values for the localization angles of all the targets involved.
It has previously been usual to establish the antenna diagrams for each individual sensor at the factory prior to commissioning of the sensor. When the radar sensor is installed in a motor vehicle, for example behind a bumper or behind a relief structure such as an emblem of the motor vehicle brand, distortions of the antenna diagram can occur and can cause systematic errors in the angle estimate. This applies in particular to the transmitting antenna diagrams for MIMO radar sensors.